docs: prepare for 3.0.0rc1

[henryiii]

Description

Prepare for 3.0.0rc1.

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📚 Documentation preview 📚: https://pybind11--5679.org.readthedocs.build/

[Copilot]

Pull Request Overview

This PR prepares the documentation for the upcoming 3.0.0rc1 release by updating the upgrade guide and the changelog.

• Updated the upgrade guide in docs/upgrade.rst to add additional context about compatibility variable warnings.
• Revised the changelog in docs/changelog.md to include the release candidate date, new features, and additional PR references.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/upgrade.rst	Modified text to clarify behavior regarding compatibility mode warnings.
docs/changelog.md	Updated release candidate details, reformatted some feature names, and added PR references.

Comments suppressed due to low confidence (1)

docs/changelog.md:35

• There appears to be an inconsistency in branch naming between 'smart-holder' and 'smart_holder'. Please confirm the correct branch name and update accordingly.

- The `smart_holder` branch has been merged, enabling

[rwgk]

Thanks a lot @henryiii!

[Skylion007]

So we aren't doing the include refactor for this release?

[henryiii]

When is it going to be ready? We've been working toward release for the last couple of weeks, with a major push during PyCon. Do you think it could be done today? (Also, the hope is to remain in RC for about a week)

[Skylion007]

When is it going to be ready? We've been working toward release for the last couple of weeks, with a major push during PyCon. Do you think it could be done today? (Also, the hope is to remain in RC for about a week)

Was hoping to get it done on/after Friday

[Skylion007]

@cyyever Might have spare cycles to help, we just need to split std.h into multiple smaller headers so people can optionally only include a subset of them.

[henryiii]

Let's get RC1 out, then this can go in during the RC period; it shouldn't affect users but we could do a RC2 if we want to make sure it gets tested. I'll add a note that this is an approved RC addition.